FIG. 1 illustrates a typical teleconferencing system 100 that includes a microphone 101, and a speaker 102 which are positioned within a room 103. The microphone and speaker 101-102 are connected to a telephone 104 which, in turn, is connected to a public telephone system network via transmitter path 105 and receiver path 106. The transmitter and receiver paths 105-106 are coupled to a telephone line interface 107 which is coupled to another telephone line interface 109, via a two wire line connection 108. As is known within such a topology, when the speaker 102 is generating audible tones, these audible tones 110 are reflected, or bounced, off of the walls, ceiling, and floor of room 103. Due to the speed of sound, these reflected audible tones do not reach the microphone at exactly the same time thus creating a received echo within the microphone 101. This type of echo is commonly referred to as an acoustic echo.
Electrical echoes are also present within the system. Electrical echoes are created when audio signals are being carried on the transmit path 105 to the telephone line interface 107. Due to the imperfections of the telephone line interface 107, an attenuated representation of the audio signals are present on the receiver path 106. As with any signal on the receiver path, if it is not canceled, will be presented to the speaker and rendered audible in the room 103.
A typical echo waveform, whether acoustic or electrical, is shown in FIG. 2. As shown, the speaker begins generation of the audible tones at t.sub.0. Due to the speed of sound, the microphone 101 does not receive the audible tones until t.sub.1. Upon initial receipt of the audible tones at t.sub.1, the bounced, or reflected, signals decay exponentially until time t.sub.2. At t.sub.2 the echo has decayed to a level that is undetectable by human ears.
To compensate for the echo of FIG. 2, many teleconferencing systems include echo cancellation circuits. One such echo cancellation circuit is described in patent application entitled APPARATUS AND METHOD FOR NOISE REDUCTION FOR A FULL-DUPLEX SPEAKERPHONE OR THE LIKE, having a Ser. No. of 07/975,348, a filing date of Nov. 12, 1992, and is assigned to the same assignee as the present invention. U.S. Pat. No. 5,410,595 utilizes two echo processing blocks. The first echo processing block provides echo cancellation of electrical echoes, while the second echo processing block provides echo cancellation of acoustic echoes. To cancel the electrical echoes, the first echo processing block estimates the amount of signal imposed on the receiver path via the phone line interface and subtracts it from the receiver path. This estimation is done by determining the transfer function of the transmit path 105, the phone line interface 107, and the receiver path 106 and utilizes the transfer function to calculate the electrical echo.
The second echo processing block, which cancels acoustic echoes, estimates the acoustic echo and subtracts it from the transmit path. To achieve this, the second echo processing block determines the transfer function of the microphone 101 the speaker 102 and acoustic parameters of the room 103. Having determined the transfer function, the second echo processing block mathematically determines the acoustic echo.
While this echo cancellation technique works well, it utilizes a fixed echo cancellation time, which, in some applications, is not optimally efficient. The technique is not optimized because, in many situations, the echo is not of a fixed duration. For example, when audible tones produced by the speaker 102 have a substantial amplitude, i.e., loud tones, the echo time may be longer than the fixed duration of the echo cancellation circuit. When this occurs, echo signals will be present on the transmit and receive paths. Conversely, if the fixed duration is set to accommodate for the loud audible tones, the fixed duration will be too long for most echo signals. When this occurs, the echo cancellation continues to perform the echo cancellation mathematical equations after the echo has decayed below an audible level, but because of the computations, the echo cancellation circuit is injecting noise into the system 100.
Therefore, a need exists for method and apparatus that can accurately determine the duration of an echo such that echo cancellation circuits may be optimized.